The Life-Cycle of Angiostrongylus cantonensis: A Parasite's Journey

Angiostrongylus cantonensis, often called the "rat lungworm," is a parasitic worm that causes a serious illness known as neural angiostrongyliasis. This illness can lead to severe headaches, nerve pain, paralysis, brain damage, and sometimes even death.

Parasites can't survive on their own; they rely on other creatures to live and reproduce. Some parasites, like Angiostrongylus cantonensis, have a complex life cycle that involves two different host species. In this case, those hosts are rats and snails or slugs.

The Rat Host:

The adult rat lungworm lives in the circulatory system (heart and lung arteries) of rats. Various types of rats, like brown rats, black rats, or Polynesian rats, can serve as the definitive host. These adult worms lay eggs in the rat's blood, which then end up in the rat's lung air sacs. These eggs hatch into tiny microscopic "baby" worms known as first-stage larvae (L1). These L1 larvae move through the rat's lungs, get coughed up, and are swallowed. After passing through the rat's digestive system, they can survive for a while in the rat's fresh feces.

The Snail or Slug Host:

Now comes the intermediate host. The rat's fecal pellets might be eaten by a land snail or slug directly, or they could dissolve in water and then be swallowed by freshwater snails. Once inside the snail or slug, the larvae find their way to the muscles, where they feed, grow, and go through two more developmental stages, shedding their skin along the way. After these changes, they become what's known as third-stage larvae (L3) and are ready to infect a rat host again. They can remain dormant and immobile in the snail or slug tissues for months.

The Transmission:

We're not entirely sure how rats get re-infected, but some probably eat infected snails directly. When snails die, the larvae may be released to water, or contaminate fresh produce. Some rats might swallow L3 larvae from the water, where they can survive for weeks. If another cold-blooded animal like a lizard, centipede, snake, or frog eats an infected snail, the larvae can survive in its body too. These animals are called paratenic hosts, and they are not necessary in the parasite's life cycle, but they can carry infectious larvae in their tissues. If a rat eats one of these hosts, the worm's life cycle continues.

Inside the Rat:

Once inside the rat, L3 larvae pass through the stomach and cross the intestinal wall into the bloodstream. They're small and vulnerable, so they quickly migrate into the central nervous system (CNS), which includes the brain and spinal cord. Over about three weeks, they grow from a tiny 1/3 of a millimeter to 1-1.5 centimeters in size. While having a worm in your brain might sound scary, this parasite has evolved to coexist with rats, and infections usually don't harm them much. Once they reach a certain size, the worms leave the CNS, return to the circulatory system, grow again, mate, and lay thousands of L1 eggs daily, contaminating the environment via rat’s feces.

The life-cycle of Angiostrongylus cantonensis.

Source: https://www.cdc.gov/dpdx/angiostrongyliasis_can/index.html

Other Hosts and Human Infection:

In some cases, the L3 larvae are accidentally swallowed by animals that don't serve any purpose in the parasite's life cycle. In these hosts, the larvae can't mature properly. In some species, like humans and other primates, dogs, and many marsupials, the migration of these larvae through the CNS can cause inflammation, affect function, lead to long-term health problems, and even be fatal. More information about human infections can be found in the section on What You Should Know About a Rat Lungworm Infection.